1. Field of the Invention
The present invention relates to an improved temperature control system for existing reflow ovens of the type used to make solder connection between surface mounted components that are positioned on a substrate. More particularly, the present invention relates to modification of controls on such reflow ovens which may be incorporated into some existing reflow ovens as well as being fitted into new ovens.
2. Description of the Prior Art
Presently, there are a large number of manufacturers of reflow ovens. The best and latest available reflow ovens, such as the Quad Profile Models designated High Thruput (HTP) have a plurality of heating zones. Each vertical zone is provided with an upper and a lower independent heating element juxtaposed each other through which passes a moving belt adapted to carry or move component populated substrates through the ovens and make solder connection of the components to the substrate, typically a printed circuit board, plastic or ceramic carrier.
When the components to be connected to the substrate are provided with leads or fingers they are inserted into vias in the substrate and may be soldered to the substrate using wave solder technique. However, when the components are to be connected using surface mounting technology (SMT), the components are not provided with leads or fingers, but are accurately placed on a conductive pattern on the substrate which has solder paste applied to pads where the electrodes, bumps or lead out terminals of the components are to be connected to the substrate.
One purpose of the reflow oven is to preheat the components and solder paste to a temperature that activates the flux in the solder paste preparing all connections for a condition that enhances making of solder connections. This stage is called preflow since it occurs prior to liquification of the micro miniature solder particles in the solder paste. If the time is too long or too short the optimum condition is not obtained. The temperature in the preflow zones is usually specified within a .+-. temperature range which is presently controlled by heater threshold setting and maintainable by on-off controllers.
Another purpose of the reflow oven is to heat the components and solder paste after being preheated to reflow temperature to effect melting and liquification of the solder particles remaining after the flux is driven off in the preheat stage. This stage is the most critical stage in a reflow oven in that the oven must have enough capacity to raise the temperature of the solder particles to a liquid state for a short predetermined critical time that may vary from a few seconds up to over one minute. The time-temperature in the reflow stage is dependent on the specification of the solder paste. There are a large number of manufacturers of a solder paste each of which sell a larger number of solder paste, each designed to produce an optimum condition for different devices and metallizations. If the temperature is raised too high, the components and the solder connection can be affected. If the time in the liquification state is too short, the quality of the solder connection is affected. If the time of liquification is too long, the lead/tin alloy begins to decompose.
So many factors are to be considered when designing a solder paste temperature profile that users of such paste have very little leeway other than to follow the advice of the paste manufacturer or obtain a second opinion which produces more desirable results.
Oven operators first set a belt speed and a manually set desired temperatures for the heat blocks according to empirical data and the solder paste manufacturers requirements. Once the heat blocks in the oven stabilize with the pre-selected threshold temperature, the substrate may be run only if the data previously accumulated contains information on the same solder paste previously run for an identical part or parts. However, the prior art reflow ovens have no way of determining if the oven or the solder paste has changed or will actually perform the same as previously.
It would be desirable to provide a method and apparatus for determining the solder paste temperature of a production component populated substrate during production in order to control a time/temperature profile which is related to obtaining optimum solder connections and yields.